Sensitive detection of analytes, such as biological analytes, continues to be a significant challenge in analytical detection methods. Frequently, detection methods require processing of multiple samples. In addition, modification of analyte molecules is often required prior to their detection. Further, analytical detection methods should be easy, rapid, and reproducible. This is particularly important when highly specialized methods and reagents, such as diagnostic methods, are unavailable.
Conventional bioanalytical methods in particular have several deficiencies. For example, hybridization of nucleic acid molecules is generally detected by autoradiography or phosphor image analysis when the hybridization probe contains a radioactive label or by densitometer when the hybridization probe contains a label, such as biotin or digoxin. The label can in turn be recognized by an enzyme-coupled antibody or ligand. Most modern biomolecule detection methods require modification of the molecule e.g. DNA or RNA or protein, making current detection methods expensive and labor intensive.
Acoustic wave sensor technology has shown broad application in detecting materials. Acoustic wave sensors detect materials by generating and observing an acoustic wave. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave. The amplitude, frequency, and/or phase characteristics of the sensor can be measured and correlated to a corresponding physical quantity.
Several different types of acoustic wave devices have been developed, but all have only limited success in measuring water soluble or biological samples. Bulk acoustic waves (BAW) propagate through a medium. The most commonly used BAW devices are the thickness shear mode (TSM) resonator and the shear-horizontal acoustic plate mode (SH-APM) sensor. Conversely, waves that propagate on the surface of the substrate are known as surface waves. The most widely used surface wave devices are the surface acoustic wave sensor and the shear-horizontal surface acoustic wave (SH-SAW) sensor, also known as the surface transverse wave (STW) sensor. All acoustic wave sensors will function in gaseous or vacuum environments, but very few of them will operate efficiently when they are in contact with liquids.
Of the known acoustic sensors for liquid sensing, the Love wave sensor, a special class of the shear-horizontal SAW, has the highest sensitivity. To make a Love wave sensor, a dielectric waveguide coating is placed on a SH-SAW device such that the energy of the shear horizontal waves is focused in that coating. A biorecognition coating is then placed on the waveguide coating, forming the complete biosensor. Successful detection of anti-goat IgG in the concentration range of ng/ml using a 110 MHz YZ-cut SH-SAW with a polymer Love wave guide coating has been achieved [E. Gizeli et al. 1997. “Antibody Binding to a Functionalized Supported Lipid Layer: A Direct Acoustic Immunosensor,” Anal Chem, Vol. 69:4808-4813.].
A comparison between different SAW sensors has recently been described (Biomolecular Sensors, Eds. Electra Gizeli and Christoffer R. Lowe (2002). They describes a 124 MHz Love wave sensor have a sensitivity of 1.92 mg/cm2. The use of SAW sensors for detection of biological compounds been reported in, for example, U.S. Pat. No. 5,478,756, WO9201931 and WO03019981, each of which is incorporated herein by reference in its entirety.
Conventional SAW devices are a poor choice for liquid detection, as the vertical component of the propagating wave is suppressed by the liquid-air barrier. One acoustic wave sensor that function in liquids is a shear-horizontal SAW sensor. If the cut of the piezoelectric crystal material is rotated appropriately, waves propagate horizontally and parallel to a liquid surface. This dramatically reduces loss when liquids come into contact with the propagating medium, allowing the SH-SAW sensor to operate as a biosensor. Many efforts at detecting liquid solution analytes (such as biological molecules) have focused on defining the interaction between the acoustic wave and the properties of the solid/liquid interface, as well as designing higher frequency SAW devises operating in the GHz range.
The present application provides a solution to the inability of SAW devices to measure analytes, including biomolecules, in liquids.